In a conventional crankset, the vertical position of the cranks in a same radial plane constitutes a dead centre position, because the weight of the person pedalling cannot produce a torque on either of the two cranks in this position. In order to overcome this drawback, it has been proposed for more than 60 years to put the toothed chain ring, which drives a bicycle chain, forwards in an offcentre position, to separate the cranks from each other and from the chain ring in order to allow them mutual angular play, and to connect each crank or the spindle thereof to the chain ring by an articulated linkage able to absorb the radial movements due to the eccentricity. FR Patent Nos. 763,303, 905,476, 984,583, 2,526,392 and 2,584,671, and U.S. Pat. Nos. 4,159,652, 4,772,252, 4,816,009 and 5,067,370 disclose such cranksets without a dead centre position. Patent application WO 95/19909 also discloses such a crankset, further including an oscillating support which carries the crank spindles to allow the eccentricity value to be modified between zero and discreet different values, when it is moved by means of a control lever.
However, none of these cranksets has made any commercial impact, probably because of manufacturing problems and problems of water and dirt resistance, since sealing is difficult to assure because of the multiplicity of moving parts.
For conventional gear and crank apparatus with a single spindle, known examples of annular sealing gaskets are illustrated in GB Patent No. 2 135 399 and FR Patent No. 2 616 493.
In a crankset of the kind indicated in the preamble, disclosed in Patent application WO 86/05459, each of the two articulated linkages between the corresponding spindle and the chain ring acting as output member includes a radial arm made integrally with the spindle and oriented in the same direction as the corresponding crank. In order to transmit an approximately tangential force and thus a torque to the chain ring, the linkage includes either a roller carried by the chain ring and engaged in a radial slide of the arm, or an oscillating disc housed in the arm and provided with an eccentric pin engaged in the chain ring. This articulated mechanism is protected, on the outer side of the chain ring, by a circular housing fixed to the chain ring, having a frontal flange provided with a central opening for allowing the inner spindle carrying the crank on that side to pass. This opening is large enough to allow the oscillating rotating movement of the housing with respect to the spindle, because of the eccentricity; it is obturated internally by a flange integrated with the spindle, a sealing gasket in the shape of an annular disc being placed between the two flanges so as to slide onto one of them. A portion of the sliding surface on this flange is thus sometimes covered by the gasket and sometimes exposed to the open air. Dirt which reaches this surface can thus be introduced into the joint, rapidly deteriorate the sealing, reach the bearings and damage them, which has led to marketing of this type of crankset being abandoned. The latter also had the drawback of the transverse space requirement of the housing containing the two radial arms between the chain ring and the corresponding crank, requiring the other crank also to be moved away to maintain the symmetry with respect to the median plane of the bicycle.